Alloy



June 18, 1935.

M. A. HUNTER 2,005,423

ALLOY Filed Jan. 18, 1953 a Q E 1.5

.0 /.0 1.5 2,0- 35 3.0 0.5 (a/0'0? added i /1.

500' 7 I I s, We

Per Cenf Ca/cl'um fies/due l-NVENTOR ATTORNEYS of wire used asresistors.

Patented June 18, 1935 UNITED STATES PATENT OFFICE ALLOY ApplicationJanuary 18,

3 Claims.

This invention relates to nickel alloys and more particularly tonickel-chromium alloys and nickelchromium-iron alloys used in electricalwork.

Such alloys are employed for the manufacture I have found that the lifeof such wire, when submitted to high temperatures is dependent, to alarge extent, on the crystal structure of the wire. I have found thatthe hours of life of nickel-chromium and nickelchromium-iron alloys maybe materially increased when certain deoxidizers are added to the meltin amounts of an entirely different character than the amountsheretofore employed. While such deoxidizers have been added to alloys ofthis character in the past, the results obtained are not comparable withthe results which I have obtained. This is probably due to the fact thatmetallurgists have ordinarily considered these additions solely from thestandpoint of increased malleability of the material and have added suchother metals in proportions calculated to produce increasedmalleability. Thus, in the prior patent to Pilling No. 1,824,966 grantedSeptember 29, 1931, the addition of calcium and other metals of thealkaline earth-metal group to a nickel alloy is disclosed but theinvention is directed sole- 1y to improving the ductility, malleabilityand working properties of such alloys.

In preparing such alloys, there are always certain impurities present inthe metals from which the alloy is formed. They also acquire variousimpurities during the melting process, particularly oxides of one ormore of the metals used in the preparation of the alloy. They possiblyalso acquire other impurities during the reduction of the metal to thefinished form of wire or strip. In the preparation of alloys containingchromium, oxides are introduced into the alloy with the chromium orferro-chromium used in the preparation of the alloys. I have found thatthe presence of oxides or sulphides contributes more to the breakdown ofthe wire than the presence of any other impurities. This is probably dueto the fact that these oxides and sulphides collect along the grainboundaries of the crystals formed by the freezing of the molten metal.When such wire is heated for prolonged periods, these segregations orcollections of impurities between the grains of the crystals are thepoints of attack. I have found that this condition can be overcome andthe life of the wire materially increased by adding a deoxidizer to thebath in quantities exceeding the stoichiometric proportions necessary toreduce the oxides present whereby a residual quantity of deoxidizer willbe present in the'fin'al product. In carrying out my invention, I employknown deoxidizers, such as metals of the alkali group, metals ofthe-alkaline earth-metal group or aluminum, silicon or magnesium, but Iemploy these metals in definite proportions for the purpose of 1933,Serial No. 652,259

achieving the desired result. The deoxidizer must be added in an amountthat will not only be sufficient to reduce substantially all the oxidespresent in the melt, but that will provide an excess of the xidizerwhich will remain in the alloy. It is im 'sible to remove all of theoxides from the bat because an equilibrium mixture between the oxidesand the deoxidizer is eventually obtained. However, the oxides can bereduced to a point where the amount present is soluble in the metals ofthe alloy in a solid state, and the formation of grain boundarysegregations thus prevented. My invention contemplates the use of thedeoxidizer in excess of the quantity required to prevent the formationof grain boundary segregation, whereby some of the deoxidizer remains inthe metal. The amount of deoxidizing agent to be added may be determinedby analyzing the constituents of the alloy and ascertaining thepercentage of oxides and sulphides present. The method of oxygen removalis in all cases identical. For example, calcium will reduce a metaloxide present in the melt according to the following equation:

The effectiveness of the deoxidizer is directly related to the heat offormation of the oxide formed. Thus, the heat liberated in the formationof calcium oxide is greater than that liberated in the formation of anequivalent quantity of oxide of silica. The addition of calcium willtherefore reduce the oxygen content of the bath to a lower concentrationthan will the addition of silicon or less effective deoxidizer. It isimpossible, however, to remove all of the oxide from the bath but thequantity can be reduced to a point where the amount present is notsufficient to collect along the grain boundaries of the crystals andthus materially reduce the life of the wire when subjected to hightemperatures.

By the ordinary assumption that the amount of deoxidizer which must beadded is the theoretical amount necessary to react with the oxidespresent, a graphic illustration, such as is shown in Fig. 1 of thedrawing, would produce a straight line. Thus, in the reduction ofchromium oxide the reaction procceds according to the followingequation:

CrzO3+3Ca -:2Cr+3CaO v Plotting the concentration of chromium oxide andgen-6mm. present wherein the chromium oxide in the bath is designated inpercentage on the left hand axis, indicated at l, and plotted againstthe calcium added, indicated at 2, the additions of calcium would berepresented by the straight line 3. However, the oxides in the bath cannever be completely eliminated and the quantity of deoxidizer to beadded to substantially eliminate the oxides should be calculated in thefollowing manner: If a. represents the original concentration inmolecules of chromium oxide or other oxides. b the originalconcentration in molecules of calcium added to the bath, and :c thereduction in concentration of chromium oxide or other oxides whenequilibrium is reached, the equilibrium constant of the reaction isrepresented by the following equation:

t eQY Where Ke is the equilibrium constant at a given temperature andthe bracketed symbols represent the concentration of the respectiveconstituents. The chromium, nickel or iron in the bath may be assumed tobe constant since the small amount of chromium, nickel or iron producedby reduction of the oxides will have little effect on the concentration.The equation may then be written:

on (ax)(b3:r)

which may be transformed into:

This equation may, in turn, be transformed to:

When this factor is taken into consideration, a

curved line 4 is obtained. The chromium oxide,

content can not be actually reduced to zero but it approaches this pointwith increasing contents of calcium in the bath. To reduce the oxides tothe lowest possible point, it is therefore necessary that the bathcontain free calcium or other deoxidizer necessary to reduce anychromium, nickel and iron oxides present in an amount in excess of thestoichiometric proportions. The greater the concentration of calcium,the lower will be the concentration of chromium oxide in the resultingalloy.

In the preparation of the alloy, the calcium or other deoxidizer is,therefore, added in quantities greater than that represented by theequation:

The amount of deoxidizer added should be such that the residualdeoxidizer left in the bath is not sufficient to impair the workabilityor other desirable quantities of the final product. Both chromium oxideand calcium are soluble in the metal bath, but their solubilitydecreases when the metal freezes. If the concentration of either exceedsthe solid solubility, the excess will be precipitated out of the alloyand exist as a separate phase. This precipitation usually takes place onthe grain boundaries through the formation of eutectics composed of themetal oxides with some of the metal of the bath. As this result isundesirable. the quantity of deoxidizing material added to the bath mustbe controlled to avoid this result.

In Fig. 2 of the drawing I have plotted the hours of life in anaccelerated test generally used in determining the relative efficiencyof wires adapted to be used for electrical heating against the residualcalcium content. The alloy in all of the tests was an alloy composed of80% nickel and 20% chromium. The hours of life are plotted on the lefthand axis indciated at 5 against the residual calcium content indicatedin percentage at 6 producing the curve I. The test used was theTentative accelerated life test for metallic materials for electricalheating of the American Society for Testing Materials described involume 29 of the Proceedings of the Thirty-second Annual Meeting of theAmerican Society for Testing Materials beginning on page 613. It will benoted that additions such that lessthan .03% of calcium remain in thealloy have very little effect on the life of the wire and that with suchadditions the nickel-chromium alloy lasted less than 100 hours. Between.03 and .1 residual calcium has a marked effect on the life of the wireand the hoursof life are increased from less than 100 hours to more than350 hours. Further additions result in slightly longer life with theresult that the life of the wire can be increased to about 400 hours.

I have also been able to increase the hours of life of similar alloys byadditions of other deoxidizers within the limits set forth above. Asstated above, various metals of the alkali group, such as sodium andlithium may be employed. In addition to calcium, other metals of thealkaline earth group, such as barium and. strontium may be employed andI use other metals, such as magnesium, aluminum, silicon, manganese orberyllium. Similar tests conducted with magnesium have resulted in theproduction of wire with a life of 400 hours when tested in the mannerdescribed above.

The absence of oxides on the grain boundaries of the alloy can also bedetermined by etching. If the oxides are present and form grain boundarysegregations, a difference in potential exists between the material onthe grain boundary and that in the body of the crystal. This differenceof electrical potential brings about a solution of the material on theboundary and the outline of the respective crystals can clearly be seenby the use of a magnifying glass by using suitable magnification. If noprecipitation on the grain boundary has occurred, the material etcheswith great difficulty or not at all. This test is employed by placingthe alloy in a solution of Marbles etch consisting of hydrochloric acid,water and copper sulphate in proper proportions. The wire is permittedto remain in the solution for determined periods of time and thenexamined under the magnifying glass to determine the effect of thesolution on the wire. An alloy of 80% nickel and 20% chromium containing.09% residual calcium after remaining in the solution for 26 seconds,shows no segregations at the grain boundaries when examined under themicroscope.

As stated above, the oxides of the metals, chromium, nickel and iron,are soluble to a limited extent in the alloys in the solid phase and.the presence of minute quantities of these oxides does not affect thecrystal structure because they are dissolved in the alloy and. notpresent alone at the grain boundaries of the crystals. Likewise, alimited amount of the deoxidizing metal, such as calcium or magnesium,does not affect thesubstantially uniform crystal structure because italso is dissolved in the metal of the alloy. The oxide of thedeoxidizing metal, such as calcium oxide or magnesium oxide, which isformed in the reduction of the oxides of the metals of the alloy, risesto the top of the bath and is removed with the slag.

In the specification the terms alloy of nickel and chromium, alloycomprising nickel and chromium and nickel-chromium alloys are intendedto cover not only alloys, the essential constituents of which are nickeland chromium,

3 but also alloys containing iron in substantial proportions.

I claim:

1. An electric resistance element consisting" essentially of nickelsubstantially 80 percent, chromium substantially 20 percent and from .03to .2 percent calcium.

2. An electric resistance element consisting essentially of nickelsubstantially 80 percent, chromium substantially 20 percent andsubstantially' .09 percent calcium.

3. A wire for use in electric heating elements formed-of an alloy ofsubstantially 80 percent of nickel, substantially 20 percent of chromiumand from .03 to .2 percent calcium.

MATTHEW A. HUNTER.

